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Detailed Description

The kernel, and various user-space applications, can automatically load kernel modules necessary to handle certain actions (e.g. flash dish with a specific filesystem is attached, other hardware is plugged in, application uses a network protocol). This may allow even unprivileged users to cause loading kernel modules, and the kernel code may be vulnerable. This feature allows system administrators in high-security situations to limit the modules loaded by modprobe to a specific list of modules configured by the administrator, making it impossible for unprivileged users to exploit vulnerabilities in modules that are not ordinarily used by e.g. attaching hardware.

modprobe can also run specified commands instead of loading a module (using the "install" configuration directive); this is restricted using the same whitelist as well.

To help system administrators compile the whitelist, additional functionality is added to modprobe: it will be possible to log all information (as if using "modprobe -v") to a specified file, including modprobe actions run in the dracut initrd. A script will be provided that compiles a proposed whitelist from the logged data.

Benefit to Fedora

If desired and configured by the system administrator, a significant reduction of the kernel-space attack surface, avoiding risk of vulnerabilities in rarely-used kernel-mode code.

A sample desktop Fedora 12 system currently has 79 modules loaded, out of 1964 available modules (4%). When counting code size, and the main kernel file (/boot/vmlinuz*) is included, the sample desktop system runs 8.36 MB of kernel-space code, out of 34.66 MB available (24%).

Scope

New features in modprobe and a very small addition to dracut (to copy the log data from the initrd to the root filesystem).

How To Test

No special hardware needed.

Configure modprobe to log its actions, reboot the system and use it as usual.

Disable the logging, generate a whitelist using a provided script.

Reboot again, verify system works correctly. Verify other modules can't be loaded.

Regenerate the initrd (to embed the whitelist into the initrd), reboot, verify the system still works correctly.

Using dracut's debugging facilities, verify the whitelist is enforced even in the initrd.

User Experience

No impact by default. System administrators who want to build a whitelist can do so, following documentation that will be provided.

Dependencies

Upstream acceptance in module-init-tools.

Contingency Plan

None necessary.

Documentation

To be written and included in module-init tools:

Updates to modprobe.conf(5)

New man page for module_whitelist_template(8)

New README.whitelist in /usr/share/doc/module-init-tools-*

Release Notes

(in "Security" section)

Fedora now allows system administrators to restrict the modules that can be automatically loaded into the kernel to a specific subset of explicitly chosen modules. This prevents attacks on vulnerable kernel code that is not necessary for ordinary system operation (e.g. by plugging in a flash drive with an unusual filesystem, by plugging in unusual hardware, initializing network connections using an unusual protocol). See /usr/share/doc/module-init-tools-*/README.whitelist for details.